SPDM defines the messages exchange protocol to do the following tasks
The specification-defined message exchanges enable Requesters to: Discover and negotiate the security capabilities of a Responder. Authenticate or provision an identity of a Responder. Retrieve the measurements of a Responder. Securely establish cryptographic session keys to construct a secure communication channel for the transmission or reception of application data.
In English, SPDM allows requester to know the capability of responder, encrypt/decrypt stuff.
The message protocol from SPDM
The message format is defined as follows
One example for GET_VERSION request and respond
section 10 of SPDM specs defines the rest of message format